A little preliminary building must be done to download libraries and build generated code, to do so:
Windows:
Open the command line.
cd to the root of the daffodil git repository. Note the default is "C:\Users\UserName\git\dfdl".
Run the following command:
sbt update-classifiers
If there was a failed resolution, re-run the above command.
Linux:
Open the terminal.
cd to the root of the daffodil git repository. Likely "/home/username/git/dfdl"
Run the following command:
sbt update-classifiers